Use of yttria-magnesia mixtures to produce highly transparent sintered alumina bodies

ABSTRACT

By using a combination of magnesia and yttria, or their precursors, in small amounts as sintering aids for alumina, sintered bodies having very high in-line transmission properties are produced. In addition the sintering time and/or temperature required to achieve a given inline transmission is reduced as compared with using only magnesia as a sintering aid.

United States Patent 1191 Mata et a1. 1 1 Jan. 16, 1973 54] USE OFYTTRIA-MAGNESI A 3,311,482 3/1967 Klingleretal. ..106/65 MIXTURES oPRODUCE HIGHLY 3,459,564 8/1969 Lachman 106/6 5 TRANSPARENT SINTEREDALUMINA FOREIGN PATENTS OR APPLICATIONS BODIES I J 740,956 8/1966 Canada..106/65 [751 Invemms F PK Tokyo; Y Tda1 867,833 5/1961 Great Britain...l06/65 Hm Takanobu Now, 1,027,939 4 1966 Great Britain ..106 65Yokohama; Chieko Yamazaki, 9 Tokorozawa-shi, all of Japan OTHER LIC NS[73] Assignee: Hitachi, Ltd., Tokyo, Japan Y. S. Kim et a1.,Theoretically Dense (99.9%)

v Polycrystalline Alumina Prepared from Cryochemi- [22] F'led: 1969cally Processed Powders," an article appearing in the 21 APPLNQ; 49 44June 1971 issue of the Ceramic Bulletin at pages 532-535. 1 J. E. Burke(Ed), Progress In Ceramic Science, Vol. Forelgn Apphcatwnvpnonty 3,1963, The MacMillan Company, New York, pages Aug. 19, 1968 Japan..43/58557 52 11.5. CI. ..264/65, 106/58, 106/62, Primary Arnold I I106/65, 264/66 Assistant Exarrriner-JohmH. Miller 51 1nt'.Cl...'..C04b/10, 0041) 35/04, c041) 35/50 Almmekcra'gr Ammem and 58 F' id is h,,.....264 65, 66, 56; 106 65 1 l [57] ABSTRACT [56] R f r n s Cit Byusing a combination of magnesia and yttria, or their'precursors, insmall amounts as sintering aids for UNITED STATES PATENTS alumina,sintered bodies having very high in-line trans- 3,377,176 4/1968w61k6d6rr 61 a1 ..264/65 mission Properties are produced- In additionthe 3,607,436 9/1971 Charleset a1 .;...106/65 tering time and/ortemperature required to achieve a 3,026,177 3/1962 Pierre ct a1. I..106/39 given inline transmission is reduced as compared with 3,026,2103/1962 Coble.... 106/39 using only magnesia as a sintering aid,3,088,832 5/1963 Somers.. ..,..106/39 3,244,539 4/1966 Hare 106/65 17Claims, 4 Drawing Figures 1 I POWDER 1101//15 sowr/a/v 0F ADM/vCOMPRESSION FINISH/N6 1 REAL sl/v fEP/NG PRODUCT 1 PATENTEDJAN 16 msSHEET 2 [1F 2 FIG. 2

06 r 03 (Wt.

1k v ZQWQEWEQQR MZZ EN FIG 4 TN v ZQWQEWEQER m2: 1 EN m a v1 -w 5 0 w wm 0 -M) r m r 0 I 18M 0 R -9 E M 0 A X 6W 1%C zil ran/J0 OW o 2 0V 1 n 0w w w m 0 INVENTOR (,Ya a Too/l, TAKANdBu a e AKINORI MuT and cHIE K0YAMAl-AKI W M W I ATTORNEY) USE OF Y'ITRIA-MAGNESIA MIXTURES TO PRODUCEHIGHLY TRANSPARENT SIN'IERED ALUMINA BODIES The present inventionrelates to a highly transparent alumina sintered body and to a processfor preparing the same.

In order to obtain a highly transparent alumina sintered body, it is ofgreat importance to carry out operations so that the resulting sinteredbody will contain a small amount of pores. Thus, research has beenconducted to find elements which, when added to alumina, may inhibit thediscontinuous grain growth of alumina and facilitate the escape ofpores, with regard to sintering. Compounds such as oxides and salts ofBa, B, Co, Cu, Fe, Hg, Mn, Ti, Mo, Zr, Y and Zn have been examined foruse in supplying these elements.

Recently, there have been developed processes for obtain-ing arelatively highly transparent alumina sintered body by compacting analumina powder of high purity incorporated with a small amount ofmagnesium oxide and then sintering the thus compacted powder at anelevated temperature.

These processes may be carried out, for example, in the followingmanner.

1. A fine alumina powder containing magnesium oxide in an amount of atmost 0.5 weight percent is molded, and the thus molded alumina powder ispresintered for more than an hour in an oxidizing atmosphere at atemperature of from 1,000 to 1,700C and subsequently sintered in vacuumor in hydrogen atmosphere at a temperature of 1,700 to 1,900C (refer toU.S. Pat. No. 3,026,210).

2. A mixture of aluminum sulfate and a magnesium salt which can beconverted to magnesia by thermal decomposition is heated to atemperature of not higher than 1,000C to obtain a powder mixture ofmagnesia and y-alumina, and the thus obtained y-alumina contained in thepowder mixture is converted to a-alumina. The resulting powder iscompression-molded and then sintered at a temperature of from 1,700 to1,950C.

3. A substantially pure, finely-divided alumina powder iscompression-molded to form a shaped body and the shaped body ispresintered in hydrogen atmosphere at a temperature of from about 1,650to 1,750C for a period of from about 50 to 300 minutes to remove thegasl-containing pores from the body. The thus presintered body issubjected to the final sintering in a hydrogen atmosphere at atemperature of from about 1,800 to 2,000C for a period of at leastminutes to additionally remove the residual pores and to improve thetransparency of the body (refer to U.S. Pat. No. 3,026,177).

4. A fine alumina powder containing yttrium oxide powder in an amount of0.5 to 6 weight percent is molded to form a shaped body, and the shapedbody is sintered at a temperature of from 1,625 to 1,700C.

The thus sintered bodies can widely be utilized for metal gas sealingtubes of high pressure metal vapor discharge lamps, furnace materials,electronics materials and other heat-resisting and corrosion-resistingmaterials.

However, the sintered bodies prepared by the abovementioned conventionalprocesses have an in-line transmission of at most about 50 percent. Thusthe conventional processes are unsuitable for the preparation of bodieswhich require high transparency, high density and high strength, andindividually have drawbacks such as the requirement of an extremely highsintering temperature for the preparation of the bodies of high quality.

The above-mentioned process of paragraph 3, for example, requires a hightemperature of at least 1,900C for obtaining a sintered body of highdensity and high transparency, and, since the prevention of the graingrowth of alumina powder is difficult due to the discontinuous graingrowth of crystal particles, this process cannot afford a sintered bodyof high density and high transparency.

The processes of paragraphs 1 and 2 also required an extremely hightemperature for obtaining a sintered body of high density and hightransparency. With respect to the above-mentioned conventional processesof paragraphs 2 and 3, in case of employing a final sinteringtemperature of 1,700C, the in-line transmission of the obtained sinteredbody is simply not more than 10 percent.

With respect to the process of paragraph 4, a lot of pores still remainin the sintered body prepared under the most suitable conditions, andthe transparency of this body, in general, is inferior to that of otherconventional processes. Further this process has such defects in heatresistance that the heat-resisting temperature of the sintered body ofthis process using yttrium oxide is extremely low, i.e., 1,800C ascompared with that of other conventional process, i.e., about 2,000C.

The in-line transmission is herein defined as the ratio of an intensityof the radiant energy entering a given sample within a specifiedentrance steric angle, to an intensity of the radiant energytransmitting within the same steric angle as said specific entrancesteric angle among the radiant energy transmitting through said sample.

In the present specification, the in-line transmission is an averagevalue of transmittivity over a wavelength range of 320 to 1,100 my.which passes through a sample of 0.5 mm thickness, and is represented bypercentage.

The object of the present invention is to prepare a polycrystallinealumina sintered body of adequately high transparency, high strength andhigh density by carrying out the final sintering at a lower temperaturethan that of the conventional arts.

Another object of the present invention is to provide a polycrystallinealumina sintered body of higher transparency than that of theconventional arts.

Further, another object of the present invention is to prepare apolycrystalline alumina sintered body in which the grain growth ofcrystal powder proceeds very slowly even when placed under a hightemperature for a long period of time.

The present invention provides a process for preparing a transparentalumina sintered body which comprises the steps of adding a small amountof substances that can inhibit the alumina grain growth, to the maincomponent comprising at least one member selected from the groupconsisting of highly pure alumina and aluminum compounds that can beconverted to highly pure alumina by calcination in an oxidizingatmosphere, and then subjecting the mixture thereof to molding and tosintering, characterized by using, as the said substances, a smallamount of at least one member selected from the group consisting ofyttrium oxide and yttrium compounds that can be converted to yttriumoxide by calcination in an oxidizing atmosphere and at least one memberselected from the group consisting of magnesium oxide and magnesiumcompounds that can be converted to magnesium oxide by calcination in anoxidizing atmosphere.

Magnesium oxide and yttrium oxide employed in the present inventioninhibit the discontinuous grain growth of alumina crystalline powder andfacilitate the removal of pores in the alumina crystalline grains and onthe crystalline grain boundary, whereby they increase the density of theresulting sintered body. The present invention has been established onthe basis of the findings that the use of magnesium oxide and yttriumoxide in combination even in a small amount gives an excellentpolycrystalline alumina sintered body.

Almost all of the magnesium oxide added to alumina vaporizes at thesintering stage and only a trace of magnesium oxide remains in theresulting sintered body.

Yttrium oxide, in contrast thereto, remains in the alumina crystallinegrains even after the final sintering, and thus it plays a role ofinhibiting the discontinuous grain growth of thecrystalline grains, evenafter the vaporization of the magnesium oxide.

' The present invention is described in detail below with reference tothe appended drawings.

FIG. 1 is a flow sheet showing an example of the steps for thepreparation of transparent alumina sintered body of thepresentinvention.

FIG. 2 is a graph showing the relationship between the inlinetransmission of sintered bodies of the present invention and the amountof yttrium oxide and magnesium oxide added to alumina.

FIG. 3 is a graph showing the relationship between the in-linetransmission of sintered bodies of the present invention and the amountof yttrium oxide added to alumina with the constant addition ofmagnesium oxide thereto.

FIG. 4 is a graph showing the relationship between the inlinetransmissions of sintered bodies obtained under several differentconditions and the amount of yttrium-oxide added to alumina.

Raw alumina employed in the present invention is preferably as pure andas fine in particle size as possible. Ordinarily, alumina having anaverage particle size up to 0.5 p. is used.

Particularly from the view points of heat resistance and corrosionresistance to metal vapor or the like, the use of alumina which containsas little silica as possible is desired.

The employment in place of alumina of such an aluminum compoundas-aluminum sulfate which can be converted to a-alumina by calcinationmay provide substantially the same results.

As the above-mentioned additives, such soluble compounds as chlorates,fluorates or carbonates of magnesium and yttrium which can finally beconverted to the oxides thereof by calcination may preferably be used inaccordance with a wet method.

Alternatively, the powders of magnesium oxide, and yttrium oxide in assuch may be mixed with alumina in accordance with a wet or dry methodunder such conditions that the alumina can be mixed therewith thoroughlyand homogeneously.

The mixture of alumina and the additives in accordance with a wet methodis dried in such a manner as to vaporize the solvent mainly by stirringat room temperature and removing the residual volatiles completely in anair bath kept at a temperature of from to 200C.

The pressurization at the molding step is not particularly restrictedbut is preferably about 1 to 3 t/cm.

The presintering step of the present invention is intended to convertthe additives to the oxides thereof, to remove the unnecessary volatilecomponents from the mixture and, if desired, to give preferableproperties for working which may be required for the reforming to thedesired shape.

Therefore, the presintering is carried out mainly in an oxidizingatmosphere at a temperature of not less than l,000C, and heating forabout one hour is adequate. If the heating temperature of presinteringis too high, the sintering of the shaped body proceeds to a relativelygreat extent, and the above-mentioned reprocessing becomes difficult tocarry out. Thus when the re-processing after the presintering is takeninto account, the presintering temperature should be specified as beingnot higher than 1 ,300C.

Even when the re-processing after presintering is not taken intoaccount, such a high temperature as above 1,600C is not desirable.

The reason therefor is that the high-temperature heating promotes thealumina crystal grain growth and makes it very hard to remove the poreson the crystal grain boundary. Accordingly, it is sufficient to heat themixture in air at a temperature of from 1,000 to l ,600C for more than 5minutes.

The shaped body may be machined when particularly precise dimensions ofthe body are required or when the molded body has been greatly deformedduring the presintering.

The sintering step is carried out at a temperature within the range of1,600C l,900C, in a reductive atmosphere where oxygen-wanting latticedefects are liable to be introduced into alumina crystals, or in such anevacuated atmosphere that the pores in the crystal grains can easilyescape out of the crystals.

The lower the sintering temperature, the longer the sinter-ing time.That is, in case the sintering is effected at such a relatively lowtemperature as about 1,600C, a sintering time of from several hours toseveral tens of hours is necessary, whereas in case the sintering iseffected at a temperature of about l,900C, heating for 5 minutes or sois sufficient. In general, sintering is desirably carried out at atemperature of from 1,700 to l ,900C for a period of 2 to S hours.

When hydrogen is selected as the sintering atmosphere, the selection ofa temperature of higher than 1,950C is not desirable. because heating ofthe alumina shaped body at this high temperature in a hydrogenatmosphere may reduce the yttrium oxide mixed and color the shaped body.If the heating is carried out for a short period of time, the reductionis not as extensive, but too short of a sintering time makes itdifficult to produce a homogeneous sintered body.

FIG. 2 shows the results of the case where the amount of magnesium oxidewas varied from 0 to 0.5

percent and the amount of yttrium oxide from to 1.0 percent.

The sintered bodies of the example shown in FIG. 2 were prepared in thefollowing manner.

Specified amounts of aqueous solutions of yttrium chloride and magnesiumchloride were added to highly pure fine alumina powder having an averageparticle size of about 0.3 1.. The resulting mixture was thoroughlystirred to form a homogeneous slurry, dried in an air bath at about 100Cand then molded under a pressure of 2 t/cm. Subsequently the molded bodywas presintered in air at l,l00C for 1 hour, and then sintered in ahydrogen atmosphere at 1,700C for 5 hours.

The line shrinkage of the molded body was from 2 to 3 percent at thepresintering step, and from 18 to percent at the sintering step.

The thus obtained plate-like transparent alumina sintered body wasabraded to a thickness of 0.5 mm and used for the measurement of thein-line transmission.

This result shows that, with respect to all curves, the in-linetransmission of the sintered body with about 1 percent yttrium oxideaddition is almost the same as that of the sintered body without theyttrium oxide addition and the effective amount of yttrium oxideaddition is approximately l percent or less irrespectively' of themagnesium oxide addition.

Further, the amount of yttrium oxide which gives the largest in-linetransmission of the sintered body is about 0.1 percent except the caseof no addition of magnesium oxide.

Meanwhile, with respect to the amount of addition of magnesium oxide,when added singly to alumina, the addition of from 0.25 to 0.5 percentgives about 20 percent in-line transmission of the sintered body and theaddition of more than 0.5 percent gives from 6 to 7 percent thereof. Incontrast thereto, the results of the composite addition of magnesiumoxide and yttrium oxide to alumina in accordance with the presentinvention is entirely different from that of the above-mentioned casewhere magnesium oxide is singly added to alumina. The addition ofmagnesium oxide in an amount of 0.1 percent or less affords a sinteredbody of the most preferable transparency.

As mentioned above, the relationship between the amount of the yttriumoxide addition to alumina and the in-line transmission of the sinteredbody is retained in both cases where the sintering temperature is higherthan 1,700C and where it is lower than 1,700C.

The above-mentioned results clarify the existence of an effect whichcannot be obtained by the single addition of yttrium oxide or magnesiumoxide to alumina, that is, the existence of a certain geometric actioneffectively induced by the composite addition of the two compounds toalumina.

FIG. 3 shows the relationship between the in-line transmission of thesintered body and the amount of the yttrium oxide addition which isvaried within a very small range.

In this example, the average particle size of raw alumina is about 0.1u; the amount of magnesium oxide addition is 0.05 percent; the moldingpressure is 2 t/cm; the presintering temperature, time and atmosphereare l,l00C, 1 hour and air respectively; and the atmosphere of the finalsintering is hydrogen.

The above-mentioned results clarify that the yttrium oxide addition evenin an extremely small amount (0.025 percent) increases the in-linetransmission by about 10 percent more than the case of excluding theyttrium oxide addition, and that, when yttrium oxide and magnesium oxidecoexist in alumina, yttrium oxide, even if added in an extremely smallamount, exerts effective actions.

Further, the results shown in FIG. 3 clarify that the high sinteringtemperature affords a highly transparent sintered body and reduces thesintering time.

FIG. 4 shows, with respect to several examples wherein the amounts ofyttrium oxide additions differ from one another, the relationshipbetween the sintering time and the in-line transmission of the obtainedsintered body. The production condition of the sintered body shown inFIG. 4 is the same as that of the example shown in FIG. 2. Sinteringtime means the soaking after elevating to a given temperature.

As the result, in case of excluding the yttrium oxide addition, thein-line transmission of the sintered body increases almost linearly inproportion-to the heating period of sintering. The rate of increase,however, is extremely small, and the 20 hour heating simply gives asintered body having an in-line transmission of about 20 percent.

In contrast thereto, the practice of the present invention with aheating time of less than 5 hours enables the rapid increase oftransmission.

All of the above-mentioned embodiments show the cases of effecting thesintering in hydrogen atmosphere. Alternatively, even when the sinteringis effected in an evacuated atmosphere, the same transparent aluminasintered body as in the above embodiments can be obtained.

Table 1 set forth below shows the in-line transmission of sinteredbodies prepared by adding yttrium oxide in an amount varied within 0 to1 percent and magnesium oxide in an amount of 0.1 percent to finealumina having an average particle size of 0.3 u and sintering themixture in vacuum.

TABLE 1 Amount of yttrium oxide 0 0.1 0.25 0.5- 1 In-line transmission21.5 53.0 40.2 32.5 25.3

The production of sintered bodies of this example was effected in thesame manner as that of every other aforesaid embodiment, and thesintering was carried out at a temperature of 1,700C for 5 hours.

Table 2 set forth below shows the in-line transmission of sinteredbodies obtained in case of using aluminum sulfate as a starting materialin place of alumina.

TABLE 2 Amount of Y,0, added 0 0.1 0.25 0.5 1.0 In-line transmission20.5 51.0 3.52 29.3 21.8

In case of using aluminum sulfate or the like, which can be converted toalumina by calcination, as a starting material, the powder thereof ismixed with additives in a wet state, and the mixture is thoroughlystirred and heated to a temperature of to C in an air bath whilestirring to dry the mixture. Subsequently, the

mixture is heated to a temperature of 1,000 to 1,100C for 1 hour to forma powder mixture of 'y-alumina, yttrium oxide and magnesium oxide, andfurther heated to l,300C for 1 hour to form a powder mixture ofa-alumina', yttrium oxide and magnesium oxide.

In this example, the amount of magnesium oxide addition was 0.05percent; the presintering was effected in air at 1,200C for 1 hour; andthe sintering was effected in hydrogen atmosphere at l,700C for 5 hours.

The in-line transmission of sintered bodies of this example are similarto those of the above-mentioned example shown in FIG. 2. Evidently, evenwhen a material which can be converted to alumina by calcination is usedas the starting material in place of alumina, the effect due to theyttrium oxide addition can also be displayed remarkably.

As is made clear by the above-mentioned examples, the present inventionprovides an excellent effect which resolves itself into the followingthree points.

1. The present invention provides a sintered body having about percentor so high transparency than that of polycrystalline alumina sinteredconventionally prepared by the single addition of magnesium oxide toalumina.

2. The present invention, in practice, provides a sintered body havingan equivalent transparency to that of the polycrystalline aluminasintered bodies prepared by the conventional processes at a lowersintering temperature than that of the conventional processes, and,besides, shortens the heating time required for the sintering.

For example, in order to obtain a sintered body having 60 percentin-line transmission, the conventional processes require the heating at1,900C, but the present invention only requires the heating at l,700C incase of adding, magnesium oxide in an amount of 0.1 percent to rawalumina and yttrium oxide in an amount of 0.1 percent.

The fact that the present invention provides a sintered body having analmost equivalent transparency to that-of a sintered body prepared bythe conventional art is a technically excellent significance in view ofthe fact that the production of a furnace which can be run steadilyparticularly at a temperature of not lower than l,700C, is extremelydifficult.

With respect to the heating time, the 5 hour sintering at l,700C of theexample of the present invention which includes the composite additionof yttrium oxide in an amount of 0.5 percent and magnesium oxide in anamount of 0.1 percent affords a sintered body having about 30 percentin-line transparency. In contrast thereto, the hour sintering (4 timesas long as that of the present invention) of the conventional process,which excludes the yttrium oxide addition, at the same temperature asthat of the present invention nearly affords a sintered body havingabout 20 percent transparency. This fact also illustrates thesuperiority of the present invention.

3. The mechanical strength of the alumina sintered body depends on thefineness and homogeneity of the crystalline powders which compose thesintered body. The alumina sintered body of the present inventioncontains yttrium oxide which is dispersed in the alumina crystals of thesintered body. Yttrium oxide in these alumina crystals plays a role ofpreventing the grain growth of the alumina crystals. Thus, even if thesintered body of the present invention is used at an extremely hightemperature exceeding 2,000C for a long period of time, its aluminagrain growth proceeds extremely slowly. Thus, the sintered body of thepresent invention possesses an excellent durability with respect tomechanical strength. In contrast thereto, magnesium oxide of theconventional processes, when added to alumina, is mainly dispersed onthe alumina crystal grain boundary and vaporizes mostly at thesintering; step and thus it cannot prevent the unnecessary grain growthof alumina powder when the sintered body is placed at a high temperaturefor a long period of time.

As mentioned above, the polycrystalline semi-transparent aluminasintered body prepared by the practice of the present invention issuperior to that of the conventional processes, and exhibits excellentproperties when used as a metal gas-sealing tube for a high pressuremetal vapor discharge lamp, as materials for furnaces, as heat resistingmaterials for various purposes, etc.

What is claimed is:

l. A process for producing a transparent polycrystalline aluminasintered body having an in-line transmission of at least 40 percent per0.5 millimeter thickness of a radiant energy of wavelengths between 320and 1,100 millimicrons which comprises the steps of (a) preparing ahomogeneous powdery mixture consisting essentially of highly pure finealumina having a particle size conventionally suitable for sintering, atleast one member selected from the group consisting of yttrium oxide andyttrium compounds capable of being converted to yttrium oxide bycalcination in an oxidizing atmosphere, the amount of said memberselected being a small but effective amount for obtaining said in-linetransmission up to 0.25 percent by weight of yttrium oxide, and at leastone other member selected from the group consisting of magnesium oxideand magnesium compounds capable of being converted to magnesium oxide bycalcination in an oxidizing atmosphere, the amount of said other memberselected being a small but effective amount for obtaining said in-linetransmission up to 0.1 percent by weight of magnesium oxide, (b)press-molding said mixture, (c) presintering the molded product in anoxidizing atmosphere at a temperature of l,000 to 1,600C. for a periodof time from 5 to minutes, and (d) sintering the presintered product inan environment selected from the group consisting of hydrogen and vacuumat a temperature of l,700 to l,900C. for a period of time ranging fromthat sufficient to produce a sintered body having said in-linetransmission up to about 5 hours, the amount of sintering time beingless at the higher sintering temperatures.

2. A process according to claim 1, wherein the yttrium compound isyttrium chloride and the magnesium compound is magnesium chloride.

3. A process according to claim 1, wherein the presintering andsintering steps comprise presintering in air at a temperature of l,000to 1,600C. for 5 to 60 minutes and then sintering in a hydrogenatmosphere at a temperature of l,700 to 1,900C. for more than 1 hour.

4. A process according to claim 3, wherein the sintering step is carriedout in a hydrogen atmosphere at a temperature of 1,700 to 1,900C. for 2to 5 hours.

5. A process according to claim 1, wherein the amount of yttrium oxideis from 0.025 to 0.25 weight percent and the amount of yttrium compoundsis sufficient to produce 0.025 0.25 weight percent yttrium oxide and theamount of magnesium oxide is from 0.025 to 0.1 weight percent and theamount of magnesium compounds is sufficient to produce 0.025 0.1 weightpercent of magnesium oxide, based on alumina, respectively.

6. A process according to claim 1, wherein the yttrium compounds are atleast one member selected from the group consisting of yttrium chlorate,yttrium fluorate and yttrium carbonate and the magnesium compounds areat least one member selected from the group consisting of magnesiumchlorate, magnesium fluorate and magnesium carbonate.

7. A process according to claim 1, wherein said effective amount ofyttrium oxide is at least 0.01 weight percent and said effective amountof magnesium oxide is at least 0.025 weight percent.

8. A process according to claim 1, wherein said fine alumina has anaverage particle size of from 0.1 to 0.5;.L.

9. A process for producing a transparent polycrystalline aluminasintered body having an in-line transmission of at least 40 percent per0.5 millimeter thickness of a radiant energy of wavelengths between 320and 1,100 millimicrons which comprises the steps of (a) preparing ahomogeneous powdery mixture consisting essentially of aluminum compoundscapable of being converted to alumina by calcination in an oxidizingatmosphere, at least one member selected from the group consisting ofyttrium oxide and yttrium compounds capable of being converted toyttrium oxide by calcination in an oxidizing atmosphere, the amount ofsaid member selected being a small but effective amount for obtainingsaid in-line transmission up to 0.25 percent by weight of yttrium oxide,and at least one other member selected from the group consisting ofmagnesium oxide and magnesium compounds capable of being converted tomagnesium oxide by calcination in an oxidizing atmosphere, the amount ofsaid other member selected being a small but effective amount forobtaining said in-line transmission up to 0.1 percent by weight ofmagnesium oxide, (b) converting the aluminum compounds to a-alumina bycalcination in an oxidizing atmosphere, so that the resulting aluminahas a particle size conventionally suitable for sintering, (c)pressmolding the calcined mixture, ((1) presintering the molded productin an oxidizing atmosphere at a temperature of 1 ,000 to l,600C. for aperiod of time from 5 to 60 minutes, and (e) sintering the presinteredproduct in an environment selected from the group consisting of hydrogenand vacuum at a temperature of 1,700 to 1,900C. for a period of timeranging from that sufficient to produce a sintered body having saidin-line transmission up to about 5 hours, the amount of sintering timebeing less at the higher sintering temperature.

10. A process according to claim 9, wherein the amount of yttrium oxideis from 0.025 to 0.25 weight percent and the amount of yttrium compoundsis sufficient to produce 0.025 0.25 weight percent of yttrium oxide andthe amount of magnesium oxide is from 0.025 to 0.1 weight percent andthe amount of magnesium compounds is sufficient to produce 0.025 0.1

weight percent of magnesium oxide, based on the alumina, respectively.

II. A process according to claim 9, wherein the yttrium compounds are atleast one member selected from the group consisting of yttrium chlorate,yttrium fluorate and yttrium carbonate and the magnesium compounds areat least one member selected from the group consisting of magnesiumchlorate, magnesium fluorate and magnesium carbonate.

12. A process according to claim 9, wherein the aluminum compoundemployed is highly pure fine aluminum sulfate.

13. A process according to claim 9, wherein the sintering is carried outat a temperature of 1,700 to 1,900C. for 2-5 hours.

14. A process according to claim 9, wherein the aluminum compounds havean average particle size of from 0.1 to 0.5

15. A process for producing a transparent polycrystalline aluminasintered body having an in-line transmission of at least 60 percent per0.5 millimeter thickness of a radiant energy of wavelengths between 320and 1,100 millimicrons which comprises the steps of (a) preparing ahomogeneous powdery mixture consisting essentially of highly pure finealumina having a particle size conventionally suitable for sintering, atleast one member selected from the group consisting of yttrium oxide andyttrium compounds capable of being converted to yttrium oxide bycalcination in an oxidizing atmosphere, the amount of said memberselected being a small but effective amount for obtaining said inlinetransmission up to 0.1 percent by weight of yttrium oxide, and at leastone other member selected from the group consisting of magnesium oxideand magnesium compounds capable of being converted to magnesium oxide bycalcination in an oxidizing atmosphere, the amount of said other memberselected being a small but effective amount for obtaining said in-linetransmission up to 0.1 percent by weight of magnesium oxide, (b)press-molding said mixture, (0) presintering the molded product in anoxidizing atmosphere at a temperature of 1,000 to 1,600C. for a periodof time from 5 to 60 minutes, and (d) sintering the presintered productin an environment selected from the group consisting of hydrogen andvacuum at a temperature of l,700 to 1,900C. for more than 2 hours andsufficient to produce a sintered body having said in line transmission.

16. A process for producing a transparent polycrystalline aluminasintered body having an in-line transmission of at least 60 percent per0.5 millimeter thickness of a radiant energy of wavelengths between 320and 1,100 millimicrons which comprises the steps of (a) preparing ahomogeneous powdery mixture consisting essentially of highly pure finealumina having a particle size conventionally suitable for sintering, atleast one member selected from the group consisting of yttrium oxide andyttrium compounds capable of being converted to yttrium oxide bycalcination in an oxidizing atmosphere, the amount of said memberselected being a small but effective amount for obtaining said inlinetransmission up to about 0.1 percent by weight of yttrium oxide, and atleast one other member selected from the group consisting of magnesiumoxide and magnesium compounds capable of being converted to magnesiumoxide by calcination in an oxidizing atmosphere, the amount of saidother member selected being a small but effective amount for obtainingsaid inline transmission up to 0.1 percent by weight of magnesium oxide,(b) press-molding said mixture, (c) presintering the molded product inan oxidizing atmosphere at a temperature of 1,000 to 1,600C. for aperiod of time from to 60 minutes, and (d) sintering the presinteredproduct in an environment selected from the group consisting of hydrogenand vacuum at a temperature of 1,700 to 1,900C. for more than 2 hoursand sufficient to produce a sintered body having said in-linetransmission, the amount of sintering time being less at the highersintering temperatures.

17. A process for producing a transparent polycrystalline aluminasintered body having an in-line transmission of at least 60 percent per0.5 millimeter thickness of a radiant energy of wavelengths between 320and 1,100 millimicrons which comprises the steps of (a) preparing ahomogeneous powdery mixture consisting essentially of highly pure finealumina having a particle size conventionally suitable for sintering, at

but effective amount for obtaining said in-line transmission up to 0.1percent by weight of magnesium oxide, (b) press-molding said mixture,(0) presintering the molded product in an oxidizing atmosphere at atemperature of 1,000 to 1,600C. for a period of time from 5 to 60minutes, and (d) sintering the presintered product in an environmentselected from the group consisting of hydrogen and vacuum at atemperature of 1,700 to 1,900C. for 2 to 5 hours.

2. A process according to claim 1, wherein the yttrium compound isyttrium chloride and the magnesium compound is magnesium chloride.
 3. Aprocess according to claim 1, wherein the presintering and sinteringsteps comprise presintering in air at a temperature of 1,000* to1,600*C. for 5 to 60 minutes and then sintering in a hydrogen atmosphereat a temperature of 1,700* to 1,900*C. for more than 1 hour.
 4. Aprocess according to claim 3, wherein the sintering step is carried outin a hydrogen atmosphere at a temperature of 1,700 to 1,900*C. for 2 to5 hours.
 5. A process according to claim 1, wherein the amount ofyttrium oxide is from 0.025 to 0.25 weight percent and the amount ofyttrium compounds is sufficient to produce 0.025 - 0.25 weight percentyttrium oxide and the amount of magnesium oxide is from 0.025 to 0.1weight percent and the amount of magnesium compounds is sufficient toproduce 0.025 - 0.1 weight percent of magnesium oxide, based on alumina,respectively.
 6. A process according to claim 1, wherein the yttriumcompounds are at least one member selected from the group consisting ofyttrium chlorate, yttrium fluorate and yttrium carbonate and themagnesium compounds are at least one member selected from the groupconsisting of magnesium chlorate, magnesium fluorate and magnesiumcarbonate.
 7. A process according to claim 1, wherein said effectiveamount of yttrium oxide is at least 0.01 weight percent and saideffective amount of magnesium oxide is at least 0.025 weight percent. 8.A process according to claim 1, wherein said fine alumina has an averageparticle size of from 0.1 to 0.5 Mu .
 9. A process for producing atransparent polycrystalline alumina sintered body having an in-linetransmission of at least 40 percent per 0.5 millimeter thickness of aradiant energy of wavelengths between 320 and 1,100 millimicrons whichcomprises the steps of (a) preparing a homogeneous powdery mixtureconsisting essentially of aluminum compounds capable of being convertedto alumina by calcination in an oxidizing atmosphere, at least onemember selected from the group consisting of yttrium oxide and yttriumcompounds capable of being converted to yttrium oxide by calcination inan oxidizing atmosphere, the amount of said member selected being asmall but effective amount for obtaining said in-line transmission up to0.25 percent by weight of yttrium oxide, and at least one other memberselected from the group consisting of magnesium oxide and magnesiumcompounds capable of being converted to magnesium oxide by calcinationin an oxidizing atmosphere, the amount of said other member selectedbeing a small but effective amount for obtaining said in-linetransmission up to 0.1 percent by weight of magnesium oxide, (b)converting the aluminum compounds to Alpha -alumina by calcination in anoxidizing atmosphere, so that the resulting alumina has a particle sizeconventionally suitable for sintering, (c) press-molding the calcinedmixture, (d) presintering the molded product in an oxidizing atmosphereat a temperature of 1,000* to 1,600*C. for a period of time from 5 to 60minutes, and (e) sintering the presintered product in an environmentselected from the group consisting of hydrogen and vacuum at atemperature of 1,700* to 1,900*C. for a period of time ranging from thatsufficient to produce a sintered body having said in-line transmissionup to about 5 hours, the amount of sintering time being less at thehigher sintering temperature.
 10. A process according to claim 9,wherein the amount of yttrium oxide is from 0.025 to 0.25 weight percentand the amount of yttrium compounds is sufficient to produce 0.025 -0.25 weight percent of yttrium oxide and the amount of magnesium oxideis from 0.025 to 0.1 weight percent and the amount of magnesiumcompounds is sufficient to produce 0.025 - 0.1 weight percent ofmagnesium oxide, based on the alumina, respectively.
 11. A processaccording to claim 9, wherein the yttrium compounds are at least onemember selected from the group consisting of yttrium chlorate, yttriumfluorate and yttrium carbonate and the magnesium compounds are at leastone member selected from the group consisting of magnesium chlorate,magnesium fluorate and magnesium carbonate.
 12. A process according toclaim 9, wherein the aluminum compound employed is highly pure finealuminum sulfate.
 13. A process according to claim 9, wherein thesintering is carried out at a temperature of 1,700* to 1,900*C. for 2-5hours.
 14. A process according to claim 9, wherein the aluminumcompounds have an average particle size of from 0.1 to 0.5 Mu .
 15. Aprocess for producing a transparent polycrystalline alumina sinteredbody having an in-line transmission of at least 60 percent per 0.5millimeter thickness of a radiant energy of wavelengths between 320 and1,100 millimicrons which comprises the steps of (a) preparing ahomogeneous powdery mixture consisting essentially of highly pure finealumina having a particle size conventionally suitable for sintering, atleast one member selected from the group consisting of yttrium oxide andyttrium compounds capable of being converted to yttrium oxide bycalcination in an oxidizing atmosphere, the amount of said memberselected being a small but effective amount for obtaining said in-linetransmission up to 0.1 percent by weight of yttrium oxide, and at leastone other member selected from the group consisting of magnesium oxideand magnesium compounds capable of being converted to magnesium oxide bycalcination in an oxidizing atmosphere, the amount of said other memberselected being a small but effective amount for obtaining said in-linetransmission up to 0.1 percent by weight of magnesium oxide, (b)press-molding said mixture, (c) presintering the molded product in anoxidizing atmosphere at a temperature of 1,000* to 1,600*C. for a periodof time from 5 to 60 minutes, and (d) sintering the presintered productin an environment selected from the group consisting of hydrogen andvacuum at a temperature of 1,700* to 1,900*C. for more than 2 hours andsufficient to produce a sintered body having said in line transmission.16. A process for producing a transparent polycrystalline aluminasintered body having an in-line transmission of at least 60 percent per0.5 millimeter thickness of a radiant energy of wavelengths between 320and 1,100 millimicrons which comprises the steps of (a) preparing ahomogeneous powdery mixture consisting essentially of highly pure finealumina having a particle size conventionally suitable for sintering, atleast one member selected from the group consisting of yttrium oxide andyttrium compounds capable of being converted to yttrium oxide bycalcination in an oxidizing atmosphere, the amount of said memberselected being a small but effective amount for obtaining said in-linetransmission up to about 0.1 percent by weight of yttrium oxide, and atleast one other member selected from the group consisting of magnesiumoxide and magnesium compounds capable of being converted to magnesiumoxide by calcination in an oxidizing atmosphere, the amount of saidother member selected being a small but effective amount for obtainingsaid in-line transmission up to 0.1 percent by weight of magnesiumoxide, (b) press-molding said mixture, (c) presintering the moldedproduct in an oxidizing atmosphere at a temperature of 1,000* to1,600*C. for a period of time from 5 to 60 minutes, and (d) sinteringthe presintered product In an environment selected from the groupconsisting of hydrogen and vacuum at a temperature of 1,700* to 1,900*C.for more than 2 hours and sufficient to produce a sintered body havingsaid in-line transmission, the amount of sintering time being less atthe higher sintering temperatures.
 17. A process for producing atransparent polycrystalline alumina sintered body having an in-linetransmission of at least 60 percent per 0.5 millimeter thickness of aradiant energy of wavelengths between 320 and 1,100 millimicrons whichcomprises the steps of (a) preparing a homogeneous powdery mixtureconsisting essentially of highly pure fine alumina having a particlesize conventionally suitable for sintering, at least one member selectedfrom the group consisting of yttrium oxide and yttrium compounds capableof being converted to yttrium oxide by calcination in an oxidizingatmosphere, the amount of said member selected being a small buteffective amount for obtaining said in-line transmission up to 0.1percent by weight of yttrium oxide, and at least one other memberselected from the group consisting of magnesium oxide and magnesiumcompounds capable of being converted to magnesium oxide by calcinationin an oxidizing atmosphere, the amount of said other member selectedbeing a small but effective amount for obtaining said in-linetransmission up to 0.1 percent by weight of magnesium oxide, (b)press-molding said mixture, (c) presintering the molded product in anoxidizing atmosphere at a temperature of 1,000* to 1,600*C. for a periodof time from 5 to 60 minutes, and (d) sintering the presintered productin an environment selected from the group consisting of hydrogen andvacuum at a temperature of 1,700* to 1,900*C. for 2 to 5 hours.